U.S. patent application number 11/125516 was filed with the patent office on 2005-10-27 for method for operating a matrix converter and matrix converter for implementing the method.
This patent application is currently assigned to ALSTOM Technology Ltd.. Invention is credited to Lacaze, Alain, Turri, Sylvie.
Application Number | 20050237774 11/125516 |
Document ID | / |
Family ID | 32239969 |
Filed Date | 2005-10-27 |
United States Patent
Application |
20050237774 |
Kind Code |
A1 |
Lacaze, Alain ; et
al. |
October 27, 2005 |
Method for operating a matrix converter and matrix converter for
implementing the method
Abstract
A method for operating a matrix converter that includes
converting m phases of a generator into alternating voltage with n
phases of a load connected to a network, wherein n<m. The n
phases of the load are alternatingly connected via a multiple
number of controllable bi-directional switches arranged in an
(m.times.n) matrix, whereby n phases of the generator are always
connected with the load while (m-n) phases of the generator are not
connected with the load. The matrix converter may be used for at
least part of the start-up process, in particular for the initial
part of the start-up process of the generator reversely operating
the matrix converter in cycloconverter mode using power available
from the network, wherein the group of m phases of the generator
are grouped in corresponding subgroups.
Inventors: |
Lacaze, Alain; (Essert,
FR) ; Turri, Sylvie; (Port-Sur-Saone, FR) |
Correspondence
Address: |
DAVIDSON, DAVIDSON & KAPPEL, LLC
485 SEVENTH AVENUE, 14TH FLOOR
NEW YORK
NY
10018
US
|
Assignee: |
ALSTOM Technology Ltd.
Baden
CH
|
Family ID: |
32239969 |
Appl. No.: |
11/125516 |
Filed: |
May 10, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11125516 |
May 10, 2005 |
|
|
|
PCT/EP03/50808 |
Nov 10, 2003 |
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Current U.S.
Class: |
363/148 |
Current CPC
Class: |
H02M 5/271 20130101 |
Class at
Publication: |
363/148 |
International
Class: |
H02M 005/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 11, 2002 |
DE |
DE 102 52 234.0 |
Claims
What is claimed is:
1. A method for operating a matrix converter, the method
comprising: operating the matrix converter to convert m phases of a
generator into alternating voltage with n phases of a load
connected to a network, wherein n<m, the matrix converter
alternatingly connecting the n phases of the load using a plurality
of controllable bi-directional switches arranged in an (m.times.n)
matrix, wherein n phases of the generator are always connected with
the load and (m-n) phases of the generator are not connected with
the load; and operating the matrix converter in a cycloconverter
mode using power available from the network, m phases of the
generator being grouped in corresponding subgroups in the
cycloconverter mode.
2. The method as recited in claim 1, wherein the operating of the
matrix converter to convert energy from the generator is performed
so as to allow a control of the frequency and to disallow a control
of the voltage.
3. The method as recited in claim 1, wherein the operating of the
matrix converter to convert energy from the generator is performed
so as to allow a switching over from a selected connected phase of
the generator to a selected non-connected phase of the generator,
such that only natural commutations occur.
4. The method as recited in claim 1, wherein m is equal to 6, n is
equal to 3, and the operating of the matrix converter in the
cycloconverter mode includes grouping the 6 phases into 3
subgroups.
5. The method as recited in claim 1, wherein the bi-directional
switches include antiparallel thyristors.
6. The method as recited in claim 1, wherein the operating of the
matrix converter in the cycloconverter mode is performed so as to
start up the generator.
7. The method as recited in claim 6, wherein the matrix converter
is operated in the cycloconverter mode for up to a predetermined
percentage of a rated speed of the generator, and is operated to
convert energy from the generator without disconnected subgroups
from the predetermined percentage up to 50% of the rated speed of
the generator, wherein the predetermined percentage is 20 to
30%.
8. The method as recited in claim 7, wherein the matrix converter
is operated to convert energy from the generator from the
predetermined percentage up to 100% of the rated speed of the
generator.
9. A matrix converter operable in a normal mode to convert energy
from a generator and operable in a cycloconverter mode using power
from a network, the matrix converter comprising: a plurality of
controllable bi-directional switches arranged in an (m.times.n)
matrix selectably connecting m inputs with n outputs; a control
system controlling the plurality of controllable bi-directional
switches; a first device in active connection with the control
system and configured to determine the signs of current in the
inputs; and a second device in active connection with the control
system and configured to determine the signs of voltages between
the inputs.
10. The matrix converter as recited in claim 9, further comprising
a signal line connecting the plurality of switches to the control
system, wherein information concerning a switch state of the
switches is transmitted to the control system by way of the signal
line.
11. The matrix converter as recited in claim 9, wherein the
bi-directional switches include antiparallel-switched
thyristors.
12. A generator unit comprising at least one matrix converter as
recited in claim 9, and further comprising at least one generator
having a rotor and a stator including windings, and a plurality of
further switches disposed on the windings for switching the matrix
converter to the cycloconverter mode and allowing formation of
corresponding subgroups.
13. The generator unit as recited in claim 12, wherein m is equal
to 6 and n is equal to 3, wherein the plurality of further switches
includes two further switches for allowing a floating ground of the
windings to be connected during operation of the matrix converter
in the normal mode and for allowing the floating ground to be
disconnected into 3 disconnected subgroups, each of the subgroups
including 2 connected generator phases.
Description
[0001] This patent application is a continuation of International
Patent Application No. PCT/EP2003/050808, filed on Nov. 10, 2003,
which claims priority to German Patent Application No. DE 102 52
234.0, filed on Nov. 11, 2002. The entire disclosure of both
applications is incorporated by reference herein.
[0002] The present invention relates to the field of power
electronics and in particular to power generation with a
synchronous generator which is operated above the synchronous mains
frequency, as well as the drive of variable-speed synchronous
motors and induction motors. Particularly, the present invention
relates to a method for operating a matrix converter which when
being operated to convert m phases of a generator into alternating
voltage with n (n<m) phases of a load connected to a network
alternatingly connects the n phases of the load via a multiple
number of controllable bi-directional switches arranged in an
(m.times.n) matrix, whereby n phases of the generator are always
connected with the load while (m-n) phases of the generator are not
connected with the load. In addition, the present invention relates
to a matrix converter for implementing the method.
BACKGROUND
[0003] In power generation, at a specified output, an increase of
the rotary speed of a turbine is typically associated with a
decrease in size and costs. Efficiency, too, can be improved.
Already, power generation turbines up to 70 MW are connected to
generators by way of gearing arrangements, so as to allow operation
at higher rotary speeds. As the output increases, the use of
gearing arrangements becomes increasingly difficult for safety
reasons. In such cases, the turbine is operated at synchronous
speed.
[0004] The use of a gearing arrangement is typically associated
with the following disadvantages:
[0005] a fixed transmission ratio;
[0006] a noise level above 100 db for 40 MW, and above 115 db for
70 MW;
[0007] mechanical losses irrespective of the particular load;
and
[0008] exacting requirements with regard to cooling and lubrication
with oil.
[0009] The use of static frequency converters in the form of
rectifier/inverter or the use of cycloconverters (power
electronics) represents an alternative. The following advantages
could be expected:
[0010] reduced costs of the generator in agreement with a constant
product of volume and rotational speed;
[0011] a standardised generator for both 50 and 60 Hz;
[0012] an adjustable speed which allows restoration of the
partial-load efficiency of the turbine;
[0013] reduced losses in relation to the gearing arrangement, at
least in partial load;
[0014] a substantial reduction in noise;
[0015] clean (oil-free) cooling;
[0016] no upper limit of the possible output, resulting in a
significant reduction in the cost of the turbine by keeping it
small--an option not provided by a gearing arrangement; and
[0017] use of the generator as a starter motor (in the case of gas
turbine applications).
[0018] Both in the case of power generation and in the case of
drives, a reduction in losses of the static frequency converters or
cycloconverters would bring about substantial cost savings. A
reduction of the losses would above all have a bearing on
investment costs because cooling accounts for a substantial part of
the total costs of the converter.
[0019] Furthermore, reduced cooling requirements provide the option
of keeping the electronics more compact, thus facilitating
integration of the power electronics in the electric power station
or even in the generator unit. Close integration of the power
electronics in the generator unit would provide the additional
advantage of short connection lines, shared coolant devices and a
smaller overall volume (savings in building costs).
[0020] In the field of large drives of up to several 10 MW, these
advantages also arise from the reduced losses, thus providing a
competitive advantage compared to direct mechanical drives of a
turbine.
[0021] The indirect conversion which is used in rectifier/inverters
(AC/DC/AC) is caused by generating a directed direct current or a
directed direct voltage from the three-phase source (mains in the
case of motors; generator in the case of power generation).
Subsequently, the direct current or the direct voltage is converted
back to an alternating current by means of an inverter.
[0022] An inductance (current converter) or a capacitor bank
(voltage converter) are switched into the intermediate circuit so
as to reduce the ripple component of the current or the spikes.
[0023] These days, rectifier/inverters make use of thyristors. If
natural commutation of the thyristors is possible, the losses in
the converter are reduced. However, induction motors for example,
take up reactive power. In order to make this reactive power from
the net available, it should be possible to switch off the current
in a specified arm of the converter at any desired time. In this
case there is forced commutation and thus there are increased
losses. In the electrical machine (generator or motor), the phase
currents are chopped direct currents. The armature reaction does
not rotate at constant speed and amplitude but instead jumps around
according to the commutation cycle. A 6-pulse or 12-pulse converter
provides six or twelve different angular positions for the armature
reaction. This results in strongly pulsating torques and large
additional losses in the electrical machine which can lead to
deterioration of the machine. In 12-pulse converters the effect is
4 times smaller than in 6-pulse converters.
[0024] Voltage converters use GTOs with their inherent high
switching losses, as well as IGBTs or IGCTs. The power of the
individual components is less than that of thyristors,
consequently, a larger number of components are required for a
specified voltage or a specified current. Voltage converters can
benefit from the use of pulse-width modulation techniques which
improve the shape of the current curves and reduce the harmonics.
The higher the switching frequencies the better, except with regard
to losses and dielectric fatigue. The curve shape of the current
can largely be sine-shaped so that a decrease of power of the
electrical machine is avoided.
[0025] Direct conversion (AC/AC) is for example possible by means
of a so-called cyclo-converter. Direct conversion provides
significant advantages from the point of view of the electrical
machine, because the current is more or less a sine-shaped wave
rather than chopped direct current. It reduces the losses which
occur additionally within the electrical machine and it also
prevents pulsating torques.
[0026] However, the use of cyclo-converters limits the achievable
frequency range to 0-1/3 of the input frequency. Due to imbalanced
operation, exceeding the 1/3 limit results in overdimensioning up
to a factor of 3.
[0027] Another possibility of direct conversion is provided by a
so-called matrix converter in which each phase of a multi-phase
source (generator or mains) is connected or connectable with each
phase of a multi-phase load (mains, passive load, motors, etc.) by
a bi-directional switch (see e.g. N. Mohan et al., Power
Electronics, 2nd Edition, John Wiley & Sons, New York pp
11-12). The switches consist of an adequate number of thyristors to
withstand the differential voltage between the phases, and the
phase currents, and to allow current reversal. They can be regarded
as truly bi-directional components with the options of jointly
using additional wiring such as snubbers or the power supplies for
the drive pulses for the antiparallel components.
[0028] The switches are arranged in an (m.times.n)-matrix at m
phases of the source and n phases of the load. This provides the
option of establishing any desired connections between the input
phases and the output phases; however at the same time it has the
disadvantage in that certain switching states of the matrix must
not be allowed since otherwise for example a short circuit would
result. Furthermore it is desirable to carry out commutation from
one phase to another phase such that the lowest possible switching
losses result.
[0029] U.S. Pat. No. 5,594,636 describes a matrix converter and a
process for its operation in which commutation between the phases
is partly carried out as a natural commutation, with a forced
commutation where natural commutation is not possible. Although
with this type of selection, switching losses are reduced due to
natural commutation, those switching losses which arise from forced
commutation still remain. Furthermore, the possible forced
commutation necessitates the use, in all positions on the matrix,
of components which can be switched off. This considerably
increases the switching expenditure.
[0030] However, it is possible to operate a matrix converter in a
way that only natural commutations are being used. This can be
achieved by only allowing the switching over from a selected
connected phase of the generator to a selected not connected phase
of the generator only if certain conditions are met. However, this
mode of operation allowing a cheap and reliable control of the
matrix converter can only be used to control frequency but not to
control the voltage. Voltage is therefore controlled by means of
the excitation system, as usual in large power generation. This
voltage control strategy does not cause any problem when the
rotating machine is operated in generating mode. But when the
rotating machine is used as a motor, torque and current control, at
low speed, requires a real voltage modulation, in-line with the
so-called constant V/f rule.
[0031] When the prime mover of the power generation unit is a gas
turbine, it is a usual practice to use a static frequency
converter, i.e. rectifier/inverters, to operate the generator as a
motor to start-up the shaft. The gas turbine does not produce a
positive torque before it has reached 40-50% of the rated speed. So
while to provide a matrix converter based on fully natural
commutation has various advantages, it still necessitates to
provide additional means to start-up the gas turbine unit in
particular in a range between 0 to 20% load, where the so-called
constant V/f rule has to be met. The strong attractiveness of the
naturally commuted converter, in terms of cost, compactness and
efficiency, is consequently reduced by the need for an additional
starting device.
SUMMARY OF THE INVENTION
[0032] It is an object of the invention to provide a method for
operating a matrix converter, a matrix converter, as well as a use
of such matrix converter, which avoid the disadvantages of the
known matrix converter solutions in the sense that, e.g., for the
start-up of the turbine using the generator as start-up device,
alternatives to the use of separate cycloconverters or static
frequency converters shall be made available.
[0033] The invention includes operating the matrix converter in
"reverse mode", so to speak, as a cycloconverter. The fact that
this is possible quite easily and without any modification of the
hardware of the matrix converter by simply changing of the mode of
operation of the matrix converter and by providing means grouping
the m phases of the generator in corresponding subgroups, is
unexpected and makes it possible to use such mode of operation of a
matrix converter for starting up a gas turbine unit by using the
generator as motor and the mains as power source.
[0034] According to a first preferred embodiment of the present
invention, the matrix converter, when being operated to convert
energy from the generator to the load, only allows to control
frequency but not voltage. In particular in this case, where such
matrix converters cannot be used in a reverse mode for start-up of
a gas turbine unit directly due to the "constant V/f rule"
necessitating the control of voltage at least within the first part
of the start-up of the generator which behaves like an electrical
machine, the switching between the two modes is advantageous.
[0035] According to still another preferred embodiment of the
present invention, the method is characterised in that the matrix
converter, when being operated to convert energy from the generator
to the load, only allows the switching over from a selected
connected phase of the generator to a selected non-connected phase
of the generator, such that only natural commutations occur. Such a
matrix converter as well as a mode of its operation has been
disclosed in DE 10051222 A1 as well as in the corresponding EP
1199794 A2, the entire content of which documents shall be
explicitly incorporated by reference herein. Usually the switching
state of the switches, the connection state of the phases of the
generator, and the signs of the currents in the phases of the
generator, and the differential voltages between the phases of the
generator, are monitored or measured. At specified, preferably
periodic, points in time switch-over occurs and for switch-over one
or several phases of the generator are selected, which phases of
the generator according to the information gained from monitoring
or from measuring are not connected and fulfil the conditions. In
particular antiparallel thyristors are used as bi-directional
switches The matrix converter, as disclosed in the above-mentioned
documents, only allows the control of frequency but not of the
voltage and can therefore not directly be used in a reverse mode to
start-up the generator at least within the initial part of the
start-up. However, using the slight modification consisting in the
above-mentioned grouping of the m generator phases into subgroups
allows to operate even such matrix converter in a reverse mode at
the same time controlling frequency and voltage.
[0036] According to another preferred embodiment the generator
comprises 6 phases and the load comprises 3 phases, and for
cycloconverter mode the 6 phases are grouped into 3 subgroups. For
this particular topology only two switches are necessary for
groping the phases of the generator.
[0037] Further preferred embodiments are described in the
claims.
[0038] The present invention additionally concerns the use of a
method as described above for starting up of a generator. More
particularly, for starting up the generator i.e. the generator and
the (gas) turbine, up to 20 to 30% of the rated speed of the
generator, the matrix converter is operated in cycloconverter mode,
while from 20 to 30% of the rated speed up to 50% or even up to
100%, the matrix converter is used in its normal operating mode
without disconnected subgroups. As it is well known, only within
the first 20 to 30% of the rated speed of the generator, voltage
control is necessary while in the range above of that mere
frequency control is sufficient. It is therefore sufficient to
operate the matrix converter in cycloconverter mode in the initial
range which absolutely necessitates voltage control and to switch
to normal matrix converter mode, preferably strictly only using
natural commutations, as described above.
[0039] Further preferred embodiments of the above-mentioned use are
described in the claims.
[0040] Apart from the above, the present invention also relates to
a matrix converter for implementing the mode of operation of a
matrix converter as described above, or to be used as described
above. The matrix converter comprises a multiple number of
controllable bi-directional switches arranged in an (m.times.n)
matrix, said switches, controlled by a control system, selectably
connecting m inputs with n outputs, and is characterised in that
first means for determining the signs of the currents in the
inputs, and second means for determining the signs of the voltages
between the inputs are provided, and that the first and second
means are in active connection with the control system. Such a
matrix converter has e.g. for the case of natural commutations
only, been disclosed in DE 10051222 A1 as well as in the
corresponding EP 1199794 A2, the content of which documents shall
be explicitly included in this application also with respect to the
matrix converter itself and shall form integral part of this
disclosure. Such a matrix converter is preferably characterised in
that the switches, which are preferentially comprising
antiparallel-switched thyristors, are connected to the control
system (control unit) via a signal line by way of which information
concerning the switch state of the switches is transmitted to the
control system.
[0041] Still further preferred embodiments of the matrix converter
according to the invention are described in the claims.
[0042] Last but not least the present invention also relates to a
whole generator unit comprising at least one matrix converter as
described above and at least one generator, which generator
comprises a rotor and a stator, characterised in that for switching
to cycloconverter mode switches are provided on the windings of
stator allowing to form the corresponding subgroups. In other words
the floating ground usually existing on the side of the stator
opposite to where the matrix converter is connected can be grouped
into subgroups as described above by means of simple switches
allowing to either have a common floating ground or subgroups (the
switches are then open).
[0043] In still another preferred embodiment of the present
invention, the generator comprises 6 phases and the load comprises
3 phases, wherein for cycloconverter mode, the 6 phases are grouped
into 3 subgroups. In this case preferably 2 switches are provided
allowing to connect the floating ground of the windings of the
stator for normal matrix converter mode and allowing to disconnect
said floating ground into three disconnected subgroups, each of
which comprise two connected generator phases.
[0044] Further embodiments result from the claims.
BRIEF DESCRIPTION OF THE DRAWINGS
[0045] Below, the invention is explained in more detail by means of
embodiments in conjunction with the drawings, in which:
[0046] FIG. 1 of is a diagrammatic representation of the
commutation with a matrix converter comprising 6 input phases and 3
output phases as it can be used for fully natural commutation;
[0047] FIG. 2 shows the schematic circuit diagram of a
cycloconverter with 3 input phases and 3 output phases according to
a preferred embodiment of the invention;
[0048] FIG. 3 shows a schematic sketch of the phase connections in
both operating modes; and
[0049] FIG. 4 shows a schematic circuit diagram of a matrix
converter connected to six phases of the generator to be started
up, with means to operate the matrix converter in cycloconverter
mode.
DETAILED DESCRIPTION
[0050] FIG. 1 shows a schematic circuit diagram of a matrix
converter comprising 6 input phases and 3 output phases which is
designed and controlled to allow natural commutations only. Such a
matrix converter has been disclosed in DE 10051222 A1 as well as in
the corresponding EP 1199794 A2. The proposed principle can however
also be applied to matrix converters which use forced commutation.
A matrix converter as described in these documents as well as its
mode of operation as described therein shall form the basis for the
examples given here. The matrix converter 10, when being used to
convert the frequency of the voltage generated by the generator 11
to a frequency as requested by the load 12, i.e. the grid to which
the generator is connected, in a time sequence 6 connects phases
G1, . . . ,G6 of a generator 11 to the 3 phases L1, . . . ,L3 of a
load 12. The power component 13 required for it comprises 18
bi-directional switches 14 in the form of antiparallel switched
thyristors. The switches 14 are arranged in a (6.times.3) matrix. A
control system (control unit) 17 is provided for selecting the
switches 14, said control system receiving time signals from a
clock 18 (a clock frequency). The switching state of the switches
14 (ON, OFF) is monitored and in each case reported to the control
system 17 via a first signal line 20. In each instance the switches
14 are selected by the control system 17 via a control line 19.
[0051] In each of the individual phases G1, . . . ,G6 of the
generator 11, a current measuring device 15 is arranged in each
instance which reports the sign of the phase current via a second
signal line 21, to the control system 17. In addition, voltage
measuring devices 16 are arranged between the phases G 1, . . . ,G6
of the generator 11, said voltage measuring devices reporting the
sign of the respective phase difference voltage to the control
system 17 via a third signal line 22.
[0052] As disclosed in DE 10051222 A1 as well as in the
corresponding EP 1199794 A2, a commutation criterion can be derived
for commutation within the matrix converter 10, said commutation
criterion being essentially based on the sign of the product of the
phase difference voltage between the phase to be switched off and
the phase to be switched on and of the phase current in the phase
to be switched off. If this product is negative, commutation
between these two phases is allowed. Otherwise commutation is
prohibited. Commutation is triggered by the control system 17, if a
commutation is present after a specified time and if the
commutation criterion is met. Since for commutation a "free" phase
of the generator 11 is required and since in each instance certain
switches 14 must not be activated, so as to avoid short circuits,
the control system 17 must know at all times which of the phases
G1, . . . ,G6 are free, i.e. in which of the phases G1, . . . ,G6
all associated switches 14 are open, i.e. not carrying any power.
The control system 17 must also know to which of the output phases
L1, . . . ,L3 the phase which is to be commuted is switched, so as
to precisely switch on that switch which is suitable for this
commutation. The above-mentioned commutation criterion is based on
the physical premise that a natural commutation between two phases
of the generator 11 can only be carried out successfully if at the
point of time of commutation t0 the absolute value of the current
iGx of the phase Gx from which one wants to commutate, is falling,
while the absolute value of the current iGy of the phase Gy to
which one wants to commutate, is rising. This necessary condition
means that the phase to which one wants to commutate, has a higher
electromotive force than, and the same sense of direction as, the
phase from which one wants to commutate. However, since the
electromotive force can only be measured during idling, the
criterion is to be established with easily accessible or measurable
quantities.
[0053] As discussed extensively in DE 10051222 A1 as well as in the
corresponding EP 1199794 A2, one can find a commutation criterion
to select natural commutations only, which is given by:
I.sub.k.multidot.(V.sub.k-V.sub.l).multidot.K.sub.ijkl<0 (1)
[0054] with the constant K.sub.ijkl depending on the mutual
inductances of the phases of the generator and the inductance of
the load. Thus if the constants K.sub.ijkl determined by the
self-inductances and mutual inductances of the generator and the
load are known, by means of the easily measurable quantities phase
current l.sub.k and phase difference voltage V.sub.k-V.sub.l signs
it can be determined at all times whether or not an intended
natural commutation between the phases k and l of the generator can
be carried out. The condition or rule (1) only depends on the signs
of the currents and voltages, not however on their actual values.
Thus the information necessary for the commutation condition can be
obtained with very simple detectors or measuring devices.
[0055] The decision process which in the case of a matrix converter
10 according to FIG. 1 leads to selection of the switches 14, is
very simple:
[0056] First the clock 18 tells the control system 17 at what point
in time according to the desired frequency and if applicable
according to any feedback information, a new commutation is to take
place, i.e. at what point in time the phases presently connected to
the load 12 are to be replaced by other phases.
[0057] As a result of continuous monitoring of the switches 14 and
the phases G1, . . . ,G6, the control system 17 knows which phases
are free, i.e. do not carry any current, and which phases can
subsequently be safely commutated. If one or two commutations are
possible, the associated switches 14 are triggered. As has already
been mentioned above, simultaneous commutation of three phases is
avoided. Any second and third commutations (possible per se) are
postponed until they can be carried out safely.
[0058] Such a matrix converter does not have any inherent reactive
power consumption. The cyclo-converter for example, which also
carries out a direct AC/AC conversion has a very small power factor
due to the trigger delays necessary to achieve a sine-shaped
voltage. Indirect converters also display a reduced power factor
due to the margin of commutation and the magnetisation power
necessary for chopping the direct currents.
[0059] Overall such a matrix converter operated using natural
commutations only presents the following advantages:
[0060] (1) concerning the cyclo-converter:
[0061] The power factor of the converter is almost 1 instead of
0.7, so that the input power, the power of the components and the
loss power are reduced.
[0062] Irrespective of the improved power factor, the new
conversion process brings about inherent losses which are reduced
by a factor of 2, thus allowing a lighter and more cost-effective
cooling system.
[0063] The output frequency is not limited to 1/3 of the input
frequency.
[0064] The control electronics are very simple.
[0065] (2) concerning the indirect AC/DC/AC converter (static
frequency converter, rectifier/inverter):
[0066] The power factors on the input side and on the output side
are the same, so that input power and power range of the components
are minimised.
[0067] Operation is completely reversible.
[0068] There is no intermediate storage of energy, which results in
cost savings and prevention of losses.
[0069] There are no pulsating torques, only a low content of
harmonics and no decrease of power at the input due to chopped
direct currents.
[0070] Slight loss power.
[0071] A synchronous machine connected to the matrix converter 10
can be operated either as a motor or as a generator. It can be
switched from motor operation to generator operation and can thus
be used as a starter motor. Both lead and lag operations are
possible without changing the control method. Autonomous operation
is also possible in which the voltage is determined by excitation
of the generator and frequency control is divided between the
generator 11 and the converter.
[0072] However, as mentioned above, such a matrix converter does
only allow conversion of the frequency but cannot be used for
controlling the voltage. To start up the rotor by using it as a
motor, at low speed, a real voltage modulation is required, which
cannot be provided by such a matrix converter. The well known
start-up problem comes from the fact that a rotating electrical
machine is nearly a pure inductance, due to a very weak electrical
resistance. Therefore the impedance is more or less proportional to
the rotating speed. At standstill, the rotating machine is nearly a
short circuit and the applied voltage should be reduced. Up to a
substantial fraction of the rated speed, the applied voltage must
be proportional to the rotating speed (constant V/f rule). Voltage
control is useless as soon as the rotation speed exceeds 20 to 30%
of the rated speed. It is of uppermost importance to notice that
this is perfectly matching the frequency range of a cycloconverter,
which output frequency is limited to 1/3 of the source (grid)
frequency.
[0073] Let us compare the architecture of the matrix converter, see
FIG. 1, to the well known cycloconverter architecture of FIG.
2.
[0074] The number of switches is the same in both cases. The only
topological discrepancy is on the star point connection of the
rotating machine.
[0075] From an operating point of view, as shown in FIGS. 1 and 2,
the path of current is also quite similar. In these figures,
similar paths in a matrix converter (FIG. 1) and in a
cycloconverter (FIG. 2) have been marked by thick lines to show
that indeed, one can find identical paths for these two topologies.
The question however remains, how it is possible to allow the
switching from cycloconverter mode to matrix converter mode.
[0076] A careful analysis of the situation reveals that only minor
modifications are required to allow a matrix converter to be
operated in cycloconverter motor:
[0077] The sketch of FIG. 3 shows that only 2 additional switches
are required to be able to switch from one operating mode to the
other for six generator phases and three mains phases. FIG. 3 a)
shows the conditions when the matrix converter is operated in
matrix converter mode in a six phase arrangement and with a common
floating ground 25 as shown in FIG. 1. FIG. 3b) shows the two
switches S1 and S2 which are necessary to allow a "reverse"
operation of the matrix converter in cycloconverter mode. The
sketch of FIG. 3 therefore shows that only 2 additional switches
are required to be able to switch from one operating mode to the
other.
[0078] The implementation of the set of two switches is shown on
the schematic diagram of FIG. 4. In other words to allow the
switching between the two modes of operation simply two (or more in
the case of a different number of phases) switches have to be
provided on the windings of the stator to allow to disconnect the
floating ground 25, which is necessary for matrix converter
operation mode, into subgroups A,B,C necessary for a cycloconverter
mode of operation. The switches can be mechanical switches as well
as switches from solid-state. The switches have to be open for
cycloconverter mode, and for matrix conversion mode the switches
are simply closed to have a common floating ground 25.
[0079] Advantages of the Proposed Solution:
[0080] The first advantage of the proposed solution lies on the
fact that it is a very minor change, i.e. normal operation of the
matrix converter and related hardware does not change.
[0081] Another advantage is that the cycloconverter mode is well
known to the person skilled in the art and there is no need for
further development for this mode of operation (for general
information related to cycloconverters e.g. the book by Thomas H.
Barton: Rectifiers, Cycloconverters and AC Controllers, Clarendon
Press, Oxford, 1994, in particular pages 420 to 478, may be
consulted).
[0082] Furthermore, there is no stringent requirement on the
additional switching means. They can be mechanical switches or from
solid-state.
[0083] The cutting power of the switch is very low. Switches are
normally open at standstill, when voltage and current are null.
They are closed as soon as the cycloconverter mode is completed. If
one needs to switch back from matrix to cycloconverter mode, it is
always possible to switch off all the switches of the converter to
cancel all the currents within a few milliseconds.
[0084] It is thus a very cheap and reliable solution.
[0085] If one wants to use now such a matrix converter for starting
up the unit, i.e. using the generator as a motor, the following has
to be observed:
[0086] Assuming that the grid has a frequency of about 50 Hz, up to
approximately 10 to 15 Hz, when starting up the unit, the voltage
has to be controlled in a substantially linearly increasing manner
(V/f rule). This can, as described above, not be done by using the
matrix converter in its matrix conversion mode as no voltage
control is possible. However, as also described above, a
cycloconverter allows voltage control and allows the conversion of
frequency up to a frequency of 1/3 of the mains, which in this case
is given by the above-mentioned 50 Hz. Consequentially, if the grid
is used as the mains, cycloconverter mode is possible up to a
frequency of approximately 16 Hz, which perfectly matches the
present situation, as for higher frequencies no voltage control is
necessary anymore, and the rotor can be driven by the matrix
converter in its matrix conversion mode. In other words, the two
modes of operation, matrix converter mode and cycloconverter mode,
ideally match each other for the start-up of a generator, as
cycloconverter mode is only reasonably possible up to 1/3 of the
mains frequency, and at the same time allows voltage control, while
for higher frequencies, the driving of the generator does not have
to be carried out under voltage control allowing the matrix
conversion mode of operation. Therefore to provide the
above-mentioned switching means allows to avoid the need of a
separate starting device for the power generation unit making
significant savings possible.
[0087] Therefore the following start-up strategy is proposed:
[0088] Use the matrix converter as a cycloconverter up to 20 to 30%
of the rated speed. This speed is still below the one when the gas
turbine starts to produce work. From 20-30% of rated speed, up to
50%, or even 100%, the matrix converter is simply used in its
normal operating mode, where it only performs frequency
control.
* * * * *